Direct access storage devices (DASD) have become part of every day life, and as such, expectations and demands continually increase for greater speed for manipulating data and for holding larger amounts of data. To meet these demands for increased performance, the mechanical assembly in a DASD device, specifically the HDD, has undergone many changes.
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks spaced at known intervals across the disk. Tracks are further subdivided into discrete portions called sectors. In a DASD device featuring an embedded, or sector, servo system, radial position information is encoded in read-only servo sectors interspersed with data sectors. When a request for a read of a specific data sector or sectors or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the appropriate information from the disk as it passes beneath the head. In the same manner, when a request for a write to a specific data sector or sectors or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk as the appropriate sector(s) pass(es) beneath the head.
The amount of data that can be stored on a disk is governed by many well-known physical principles. One factor in determining the amount of data that can be stored in an HDD is the ability of the magnetic transducer to write closely spaced data tracks onto the disk surface. The spacing of tracks on a disk surface is known as track pitch, and the unit of measure for expressing the density of tracks on a disk is tracks per inch or TPI. A smaller track pitch results in a higher TPI. The servo system of the HDD enables in part the magnetic transducer to be positioned precisely to read or write a data track at a radius on the disk, thus enabling adjacent data tracks to be written with a high TPI.
Whether the data track is being written or read, the HDD must be able to find the track and settle upon it in a minimal amount of time. This amount of time is typically known as seek-settle time. When seek-settle time is coupled with the time for the target data sector to rotate to where it can be read or written and the time required for the HDD to transfer the data between the host computer and the disk, this total time is known as latency. An HDD user usually prefers short latency. The robustness of the servo system will determine if the magnetic transducer has settled on a desired data track or needs to make another attempt at settling on the data track.
The robustness of the servo system is affected by the ability of the voice coil motor driver to direct the voice coil motor to align the head, via the arm, over the desired specific track location. Ultimately, the quicker and more accurate the servo system is in locating a specific track location, the faster an HDD may perform, thereby reducing latency. Continuing advances are being made in the design of HDDs as more demands are made for faster HDD performance.